Water-containing coal, for example, brown coal has a high water content and a number of relatively large pores in its structure. Even if the water-containing coal is pulverized and dried for use, the size and the number of the pores hardly change. Therefore, the coal which is obtained by drying the water-containing coal has the risk of oxygen entering the pores during storage or transportation of the coal to cause slow oxidization reaction to cause spontaneous firing. Therefore, such water-containing coal is utilized in extremely limited areas near coalfields under the present circumstances.
The methods for dewatering water-containing coal, for example, brown coal by performing hydrothermal treatment for the coal at a temperature of 250 to 350° C. under a pressure of 4 to 17.2 MPa are tried (see the following non-patent documents 1 to 4). It is reported that when the hydrothermal treatment is carried out under such a pressure, the brown coal is dewatered and the volume of the pores in the coal decreases (see non-patent document 1).
However, the decrease in the pore volume is not sufficient, and the above described problem is not solved sufficiently yet. The mixture (water slurry) of the coal, which is dewatered by the above described methods, and water requires the water content which is twice to four times as high as that of the mixture of ordinary bituminous coal and water in order to obtain about the same viscosity as that of the mixture of ordinary bituminous coal and water, which is suitable for transportation, and therefore, is not economical. The treatment cost of dewatering and draining of water accompanying the dewatering is high, and therefore, these methods are not put into actual use.
non-patent document 1: “Effect of processing conditions on organics in wastewater from hydrothermal dewatering of low-rank coal” by L. Racovalis et al., Fuel, vol. 81, pages 1369 to 1378, 2002
non-patent document 2: “Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product” by George Favas, et al., Fuel, vol. 82, pages 53 to 57, 2003
non-patent document 3: “Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals” by George Favas et al., Fuel, vol. 82, pages 59 to 69, 2003
non-patent document 4: “Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals” by George Favas et al., Fuel, vol. 82, pages 71 to 79, 2003